The present invention concerns a catalyst based on a halogenated alumina, preferably chlorinated, its preparation and use in a process for the isomerisation of normal C.sub.4 -C.sub.6 paraffins.
Isomerisation of normal paraffins containing 4 to 6 carbon atoms per molecule has now assumed considerable importance in the petroleum industry, mainly because of the removal of lead alkyls from petrol.
Isomerisation of n-butane produces isobutane for the aliphatic alkylation of olefins and for the synthesis of MTBE (methyl tertiobutyl ether) by dehydrogenating isobutane, to produce an alkylate with a high octane number and MTBE respectively, which compounds can be incorporated into petrol fractions.
Isomerisation of normal C.sub.5 -C.sub.6 paraffins can transform paraffins with low octane numbers to isoparaffins with high octane numbers.
Three types of catalyst are traditionally used to carry out the isomerisation of normal paraffins containing 4 to 6 carbon atoms per molecule, preferably 5 to 6 carbon atoms per molecule:
Friedel-Crafts type catalysts, such as aluminium chloride, which are used at low temperatures (about 80.degree. C. to 130.degree. C.); PA1 catalysts comprising at least one metal from group VIII on a support based on a halogenated, preferably chlorinated, alumina, used at medium temperatures (about 150.degree. C.); PA1 zeolitic catalysts comprising at least one metal from group VIII deposited on a zeolite, used at high temperatures (250.degree. C. and more); those catalysts lead to smaller gains in the octane number of the products obtained than with the two catalyst types described above but have the advantage of being easier to use and more resistant to poisons. Nevertheless, they cannot be used to isomerise n-butane since they also produce a lower acidity than the two types of catalyst described above.
A number of patents concern monometallic catalysts based on platinum deposited on a halogenated alumina and their use in isomerisation processes of normal paraffins. An example is U.S. Pat. No. 3,963,643 which involves treatment with a Friedel-Crafts type compound followed by treatment with a chlorinated compound containing at least two chlorine atoms.
More recently, U.S. Pat No. 5,166,121 has claimed a catalyst comprising gamma alumina formed into spherules and containing between 0.1% and 3.5% of halogen on the support. An extremely small amount of halogen, preferably chlorine, is deposited on the support.